So I wrote up a quick flush method, and saved it in a file models/session.rb.

class CGI::Session::ActiveRecordStore::Session
  def self.flush_old_empty_sessions
     self.delete_all "DATE_SUB(NOW(),INTERVAL 6 HOUR) > 
     updated_at and BIT_LENGTH(data) <= 688"
  end
end

This says nuke all sessions which are over 6 hours old, and which are empty. (688 is the length of the serialized session with an empty flash)

MySQL specific, and susceptible to changes in either session structure or its serialization. But it was quick and easy and worked for me.

Then you simply need a cron job like: ruby script/runner 'CGI::Session::ActiveRecordStore::Session.flush_old_empty_sessions'